1. Field of the Invention
This invention relates to semiconductors and more particularly to MOSFET memory devices.
2. Description of Related Art
Currently a method of making a source-side injection EPROM/Flash EPROM cell is to use a one side spacer (oxide or polysilicon) which needs a critical masking step to etch one side of a spacer away.
There are several known ways of making asymmetric EPROM or flash memory structures which require critical alignment masking steps with all the known problems attendant therewith such as D. Liu, et al, "Optimization of a Source-Side-Injection FAMOS cell for Flash EPROM Applications" IEDM 91 315-318, IEEE (1991).
There is a technology called large tilted angle ion implantation which allows ion implantation to be performed at angles other than the vertical, that is 0.degree.. This technique has been used to make various MOSFET LDD integrated circuit devices. The U.S. Pat. No. 4,771,012 to Yabu et al; U.S. Pat. No. 5,073,514 to Ito et al; U.S. Pat. No. 5,158,901 to Koss et al and U.S. Pat. No. 5,147,811 of E. Sakagami all show the use of tilted angle ion implantation to make integrated circuit devices. U.S. Pat. No. 5,147,811 describes the formation of a P region under a gate using LATID and then a vertical ion implantation to form the N+ S/D regions. The publication "1/4 .mu.m LATID (LArge-Tilted-angle Implanted Drain) TECHNOLOGY" by T. Hori, published in IEDM 89 pages 777/780 and "Graded-Junction Gage/N- Overlapped LDD MOSFET Structures for High Hot-Carrier Reliability" by U. Okumura et al published in the IEEE Transactions on Electron Devices, Vol. 38, No. 12, December 1991 (pages 2647-2656) show further use of tilted angle ion implantation. These references all use single tilted angle ion implantation to improved device performance.
U.S. Pat. No. 5,073,513 of Ito et al discloses the formation of an LDD MOSFET using LATID to form the N- and then uses vertical ion implantation to form the N+ region.
An object of this invention is a process which avoids any critical masking steps needed to fabricate an asymmetric, non-volatile memory cell.
In accordance with this invention a semiconductor is formed using a Large-Angle-Implant (LAI) process to generate an off-set region for formation of an asymmetric source/drain region that can enhance the programmability by using the characteristics of source-sided injection in programming operations.